Introduction
Magnetic levitation is one example of levitation in physics. It relies only on the forces generated by magnetic fields to overcome gravity. Right now you are probably thinking, isn't levitation with magnets as simple as the repulsion that takes place between oppositely poled bar magnets? Sorry to say but the answer is no and was no ever since 1842 when Samuel Earnshaw proved that no static configuration of permanent magnets allows for stabilized levitation [1]. What do we mean by stabilization? We mean that if the object, or in this case magnet, is displaced by a small amount, a force will be there to counteract this movement and push or pull the object back to its levitating position. If you happen to have a pair of magnets on hand, try using the repulsive force to balance them yourself, as seen below. After a few tries you will see that even if one floats with respect to the other for a fraction of a second it will soon fall off.
If levitation with magnets can't be done, according to Earnshaw, then why does this page even exist? As physicists, we have found ways around Earnshaw's Theory. By using little tricks such as diamagnetic materials, superconductors, and feedback systems [2-4] stable magentic levitation has been achieved. Learning the theory behind a physics principle is especially fun when you can see it actually being used in the real world. With that being the case, instead of going through boring derivations about magnetic levitation, we will focus on an important application, high-speed trains, known as maglev trains.
What are Maglev Trains?
As you may have figured out looking at the word maglev, maglev trains are trains that use magnetic levitation. Other than airplanes, fast and commercial transportation is limited. Maglev trains may be the answer to this problem. Maglev does not only refer to the train but also the track on which they run. By using powerful magnets on both the train and the guideway, one is able to not only levitate the train which allows for higher speeds than the traditional wheel and track locomotive but also accelerate the train. To see what these trains typically look like, view the image below [5]. You can see that these trains are aerodynamic to help reduce drag.
Two common methods are currently being used to levitate maglev trains: electromagnetic suspension (EMS) and electrodynamic suspension (EDS). Prior to discussing these two methods, we will learn a little bit of physics, specifically Faraday's Law of Induction and Lenz's Law.
The Physics
Faraday's Law of Induction and Lenz's Law
In simple terms, Faraday's Law of Induction says that a magnetic field has the ability to produce an electric field, by an induced current. We say induced, because the magnetic field "makes" it. The best way to understand the phenomenon is experimentally. Imagine a loop of wire that is made of a material that allows for the flow of charge, a conductor. Now connect that loop to an ammeter such that any induced current can be measured. Initially the ammeter will read zero but what happens if a bar magnet is pushed towards or away from the loop, to within a close enough proximity? Try it yourself using the applet below.Hopefully you noticed that as you move the magnet through the loop or out of the loop that a current was produced (in the form of a voltage). Even if you place the magnet in the loop but do not move it no current is generated [6]. To summarize the important points above: 1. Current only appears when there is relative motion between loop and magnet 2. The speed of the magnet moving is related to the current 3. When moving the magnet in opposite directions, the current will flow in opposite directions (In the applet this corresponds to a positive or negative voltage). A current is generated and can flow clockwise or counter-clockwise around the loop, but how do we know its direction? To determine the direction, Lenz's Law is required. In general it says that the current produced will be in a direction that creates a magnetic field that opposes the magnetic field that caused the induced current in the first place [6]. This is best understood with an illustration [7]:
Stable Levitation of Maglev Train
As mentioned before the two main methods currently used for maglev levitation are electromagnetic suspension (EMS) and electrodynamic suspension (EDS). Electromagnetic Suspension (EMS) The first type of maglev levitation, EMS, requires two things: getting the train off the ground and stabilizing the train while it moves. The first requirement is met by using attractive forces between magnets [8]. Currently, EMS levitation practices are being applied in Germany on a system known as Transrapid [9]. To see one possible method for levitating the train, look at Transrapid's design, displayed in the figure below [10]. This system separates levitation and guidance, making the distinction easier to see.
Electrodynamic Suspension (EDS) The second type of maglev levitation, EDS, uses the consequences of Faraday's Law and Lenz's Law. Unlike EMS, EDS relies on either a combination of repulsive and attractive forces or in some cases just a repulsive force [11]. In both cases it is a result of an interaction between coils in the guideway and magnets on the train. As the on-board magnet moves relative to the guideway coils a current is induced via Faraday's Law. When using a special coil that takes the shape of the number eight, the induced current can provide both attractive and repulsive forces that levitate the train [11]. The illustrations below provide an easy way of picturing it [12].
Additional guidance coils are sometimes used but since the current in the coils depends on the position of the magnets on the train, any displacements are often self-correcting. If you understand Faraday's Law you may be asking to yourself, won't there be no current in the coils when the train is not moving? The answer is yes and because of this a minimum speed is required to get the train off the ground. At low speeds the train is equipped with wheels [8]. Propelling the Maglev Train Now that we have the train in the air, how are we supposed to move it? There are a few alternative methods for accelerating the train [8] but here we will just provide a basic idea. One method uses propulsion coils, fancy electromagnets, that when using an alternating current, one that changes in polarity after a given amount of time, can push and pull the train. The magnet that may have been initially pulling the train by attraction can push the train as it passes later by simply changing its magnetic property. Remember that electromagnets allow for this since the current can be electronically controlled. By interpreting this as coils having north or south pole properties, the illustration below [13] provides a demonstration on how a maglev train could move.
Summary
With technology consistently improving in the world it is likely that one day you will see or even ride a maglev train. If you happen to, remember that the entire system is based on the basic physics principles of magnets. High-speed movement is a result of magnetic levitation while propulsion is provided by the fine placement of electromagnets in the form of coils. Electromagnetic suspension and electrodynamic suspension are only two types of magnetic levitation and many other forms exist and even more are still likely to be developed.
